Embodiments of the invention relate generally to imaging and more particularly to contrast enhancement of time-resolved fluorescence images.
In modern healthcare facilities, non-invasive imaging systems are often used for identifying, diagnosing, and treating physical conditions. Medical imaging encompasses different non-invasive techniques used to image and visualize the internal structures and/or functional behavior (such as chemical or metabolic activity) of organs and tissues within a patient. Currently, a number of modalities exist for medical diagnostic and imaging systems, each typically operating on different physical principles to generate different types of images and information. These modalities include ultrasound systems, computed tomography (CT) systems, X-ray systems (including both conventional and digital or digitized imaging systems), positron emission tomography (PET) systems, single photon emission computed tomography (SPECT) systems, and magnetic resonance (MR) imaging systems.
Further, fluorescence imaging techniques typically utilize differences in the fluorescence response of normal tissue and abnormal tissue, such as in the detection and localization of cancer. Fluorophores that are excited during fluorescence endoscopy may be exogenously applied agents that accumulate preferentially in disease associated tissues, or they may be the endogenous fluorophores that are present in all tissue. In the latter case, the fluorescence from the tissue is typically referred to as autofluorescence. Tissue autofluorescence is typically due to fluorophores with absorption bands in the ultraviolet and blue portion of the visible spectrum and certain emission bands in the green to red portions of the visible spectrum. In tissue states associated with early cancer, the green portion of the autofluorescence spectrum is appreciably suppressed. This spectral difference between disease and healthy tissue may be used to distinguish normal tissue from suspicious tissue.
Moreover, fluorescence detection offers one of the most sensitive methods for quantification of probe molecules in biological and material systems because it can attain near single-molecule sensitivity levels. Consequently, this technique is widely used in the assaying of biochemical and cellular systems, and in particular the microscopic imaging of cell-based assays, where rich biological information is provided from multiplexed high-content data.
In addition, fluorescence imaging can enhance the contrast between different fluorophores present in tissue or exogenous fluorescent dyes used during clinical and preclinical operations. The field of view and overall speed of the imaging system are critical requirements for real-time and/or high-throughput imaging applications. Existing methods, such as time correlated single photon counting (TCSPC) or frequency domain phase sensitive detection (FDPSD) often face difficulty in acquiring and displaying the processed image data fast enough for practical use. Moreover, in conventional frequency domain lifetime imaging the modulation frequency must be changed to obtain multi-frequency phase information which may lead to longer imaging time and slower image update rates.
It is therefore desirable to have an improved multi-frequency modulation scheme for high-speed frequency domain phase sensitive detection of fluorescence and also permit rapid excitation and collection of time-resolved multi-frequency fluorescence data from a wide field of view.